A first substrate on which an optical-waveguide groove is formed and a second
substrate.
The second substrate is bonded to the plane of the first substrate on which the
optical-waveguide groove is formed by a material having a refractive index higher
than those of the first substrate and second substrate. The optical-waveguide groove
is filled with the material, and the refractive index of the first substrate is
different from the refractive index of the second substrate.